Lack of Effect of Ultrasound on the Percutaneous Absorption of Benzydamine

Ultrasound-mediated transdermal drug delivery: mechanisms, scope, and emerging trends

The use of ultrasound for the delivery of drugs to, or through, the skin is commonly known as sonophoresis or phonophoresis. The use of therapeutic and high frequencies of ultrasound (≥0.7MHz) for sonophoresis (HFS) dates back to as early as the 1950s, while low-frequency sonophoresis (LFS, 20-100kHz) has only been investigated significantly during the past two decades. Although HFS and LFS are similar because they both utilize ultrasound to increase the skin penetration of permeants, the mechanisms associated with each physical enhancer are different. Specifically, the location of cavitation and the extent to which each process can increase skin permeability are quite dissimilar. Although the applications of both technologies are different, they each have strengths that could allow them to improve current methods of local, regional, and systemic drug delivery. In this review, we will discuss the mechanisms associated with both HFS and LFS, specifically concentrating on the key mechanistic differences between these two skin treatment methods. Background on the relevant physics associated with ultrasound transmitted through aqueous media will also be discussed, along with implications of these phenomena on sonophoresis. Finally, a thorough review of the literature is included, dating back to the first published reports of sonophoresis, including a discussion of emerging trends in the field.

Scanning electron microscopic evaluation of the skin surface after ultrasound exposure

BACKGROUND

Transdermal drug-delivery systems have become widely accepted clinically for the administration of several systemic drugs. Recently, research on ultrasound irradiation to facilitate the penetration of drugs through the skin have been reported. The present study investigated the morphological changes induced in skin after ultrasound irradiation to hairless mouse skin and human skin.

METHODS

Hairless mice and human skin were immersed in an ultrasound irradiation water tank. Ultrasound was delivered for a duration of 5 min. Ultrasound frequency of 48 kHz was generated at an intensity of 0.5 W/cm2. The surface of the skin was observed with scanning electron microscopy for comparison with skin samples unexposed to ultrasound energy.

RESULTS

For hairless mouse exposed to ultrasound, the cells of the stratum corneum of the skin surface were almost completely removed. The polygonal cells of the stratum spinosum and basal cells were exposed. In addition, large craterlike pores with a diameter of 100 microns were formed sporadically in some of the skin samples. In contrast, the surface of human skin exposed to ultrasound showed only slight removal of keratinocytes around the hair follicles.

CONCLUSION

The removal of the stratum corneum and other alterations in hairless mouse and human skin by ultrasound may explain the enhancement of transdermal drug penetration. The effect on human skin was relatively minor compared with that on hairless mouse skin.